A 3D visual effect may be created by a projector system by operating the projector to deliver left and right images representing different viewpoints to a viewer (observer) who is wearing special 3D viewing eyeglasses. For example, the viewer may wear polarized eyeglasses having polarizing filters (e.g. linearly polarized eyeglasses or circularly polarized eyeglasses). In such case the left and right images are each polarized so that they can be seen by the intended eye but not the other eye when wearing the polarized eyeglasses. In other display technologies, the user may wear spectral filtration eyeglasses to view different left and right images. In such case the projector is operated to provide spectrally filtered light to the viewer so that the left eye is presented with light in a first set of spectral bands (providing a left image) and the right eye is presented with light in a complementary, second set of spectral bands (providing a right image). In each case the human visual system of the viewer combines and interprets the left and right images to perceive a single 3D image having the illusion of depth.
Some 3D projector systems use a globally controlled polarizer mounted in front of a projector. For example, FIG. 1 shows a front-view projector system 20 having a projector 22 and a reflective screen 24 which reflects the light from projector 22 toward a viewing area in front of screen 24. A polarizer 26 is positioned in the light path between projector 22 and screen 24. Polarizer 26 is controllable to alternate between two opposite polarization states in synchronization with the display of left and right images L, R by the projector. Other 3D projector systems may use an optical element to modulate the polarization of light emitted from a projector so that left and right images are alternately displayed to a viewer.
In 3D projector systems such as those described above, a design objective is to minimize crosstalk between left and right channels. Crosstalk occurs if one eye sees some residue of the image intended for the other eye. Typically, to minimize crosstalk for 3D projector systems, left and right images are alternately displayed such that the entire screen is made to appear and disappear to each eye of the viewer during each cycle. When a left image is presented to the left eye, a blank screen is presented to the right eye, and vice versa. Flickering and viewer eye fatigue may result if the refresh rate to each eye is below a flicker fusion threshold. 3D viewing is more prone to flickering than 2D viewing because the 3D refresh rate is half of the 2D refresh rate. For example, if a cinema projector is capable of displaying images at a maximum frame rate of 48 fps, then for 3D viewing the frame rate is 24 fps for each eye, which may be below the flicker fusion threshold. The flicker fusion threshold is variable from person to person and also depends on factors such as amount of modulation, intensity, image size (field of view) and brightness.
FIG. 2 shows the use of a polarizer switching between opposite left and right polarization states P1, P2 for two projector subframe images I1, I2, respectively, to generate different images to the left and right eyes of the viewer. The numbers within each image represent the light intensity. In the illustrated example, the vertical polarizer state P1 directs all the light from image I1 toward the left eye while blocking all light to the right eye. The horizontal polarizer state P2 directs all the light from image I2 toward the right eye while blocking all light to the left eye. The resulting subframe images shown to each of the left and right eyes are L1, R1 for the first subframe and L2, R2 for the second subframe. The human visual system of the viewer combines the light from the two subsequent frames for each eye so that the viewer sees time-integrated left and right eye images LTI, RTI.
As illustrated by FIG. 2, there is some reduction in the image intensity of time-integrated images LTI, RTI, from target images LTARGET, RTARGET. Also, as can be appreciated by comparing subframe images L1 to L2, and R1 to R2, the large differences in image intensity between subsequent frames for each eye may cause flicker and viewer eye fatigue.
Patent literature describing technology in the general field of this invention includes:
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The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.